diff --git a/driver-csi.c b/driver-csi.c
index 9374c55..888f78b 100644
--- a/driver-csi.c
+++ b/driver-csi.c
@@ -363,7 +363,7 @@ static TermKeyResult peekkey_ss3(TermKey *tk, TermKeyCsi *csi, size_t introlen,
 
   if(key->code.sym == TERMKEY_SYM_UNKNOWN) {
 #ifdef DEBUG
-    fprintf(stderr, "CSI: Unknown SS3 %c (0x%02x)\n", cmd, cmd);
+    fprintf(stderr, "CSI: Unknown SS3 %c (0x%02x)\n", (char)cmd, cmd);
 #endif
     return TERMKEY_RES_NONE;
   }